Were the following numbers allocated to locos withdrawn before the renumbering started? Otherwise there are unexplained gaps:

1340 to 1345 (B6 start at 1346 for no apparent reason)

1673 to 1677 (between B17/4 and B5)

Related to this why did the J94 start at 8006? They appear to be numbered after the handful of diesels the LNER had but there was no 8005 and as far as I can tell the loco that became BR 15004 never carried 8004.

Full descriptions of the above and many other changes are dealt with at length and in great detail in RCTS Locomotives of the LNER Part 1 - Preliminary Survey on pp28-42. If the answers are not there they are unlikely to be found anywhere else.

It was a Thompson-inspired scheme that, although it had its fundamental merits, was fiddled about with so much it failed to deliver the coherence originally intended. Just accept that there were failings as well as planned gaps for locomotives subsequently destroyed by enemy action, others loaned to the military but numbers allocated in anticipation of their return, earlier scrappings and jiggling to accommodate planned numbers of standard construction under Thompson's simplified standard classes regime that did not necessarily materialise, together with construction switched from one type to another for similar reasons.

The final LNER numbering scheme was much better than that of the other 'big four' companies, but I've always wondered why a lot of the class sequencing was done in such a higgledy-piggledy way (eg: the 0-6-0s and 0-6-0Ts). There were also inexplicable gaps (163 to 499, then 540 to 799). 60700 was a BR invention AFAIK.

Leaving just six numbers for diesel shunters was just silly as it was obvious that the LNER would have continued to build more had it continued.

Leaving the 6000 series vacant, apart from a few electrics, didn't make sense either.

The other companies had their problems as well, for example the LMS, which for both the class 4MT 2-6-4Ts and the class 5s had to go backwards to accommodate new locos.

I think I am right that Thompson envisaged that several hundred L1s would be built (perish the thought!), so how they would be numbered is anyone's guess.

One : what was the basis of the numbering of the Pacifics within each class? Why weren't they numbered in order of construction?

Two: how did the LNER cope with the J4s, where old and new numbers had overlapping ranges so the number and class alone wouldn't identify whether a loco had been renumbered or not.

Kudu

One - RCTS 'Greenie' Part 1 is unable to answer this so it is most unlikely that anyone is this forum can. One part of the change was a 'fiddle' by Thompson to get his new Pacifics in a number block commencing 500 instead of the allocated 200-14 for whatever reason.

Two - the J3 and J4 classes were not the only ones where numbers overlapped or were reshuffled, so careful records were kept on a weekly basis of all the changes made.

If you read the chapter titled LOCOMOTIVE NUMBERING SYSTEM on pp28 et seq you will find that there were several more anomalies of the types you query above.

Under the, "First," 1948 re-numbering scheme all of the locomotives woould have been re-numbered 500 to 578 in their, "original build," numbering sequence.

Under the, "Final," 1948 re-numbering scheme all locomotives recieved numbers 35 to 112 in the sequence of their LNER numbers [at that time]. This applies to all locomotives with one exception, 500/4470/113 Great Northern which was re-built, by Thompson, becoming Class A1/1. [Much information can be discovered regarding this re-build elsewhere........]

You are correct, under the, "First," re-numbering scheme all of the A4's would have been re-numbered 580 to 613 in their, "original build," numbering sequence. [This sequence excluded 4469 which was destroyed in a WWII bombing raid on York] However, only four locomotives ran, in service, bearing their numbers allocated in this sequence.

When they were allocated their, "Final," numbers, in the 1 to 34 sequence, instead of receiving them in, "original build," sequence, those named after, "dignitaries," came first [1 to 8], followed by the Commonwealth named loco's [9 to 13], then the, "Silver," named loco's [14 to 17] , followed by those named after birds [18 to 34]. Again..... exceptions to the rule - 4485/587/26, "Kestrel," became, "Miles Beevor,", 4487/28, "Sea Eagle,", became, "Walter K. Wigham," and, 4903/34, "Peregrine,", became, "Lord Faringdon" but still retained their allocated numbers in the, "Final, " sequence.

My last two posts specifically address the queries regarding the Gresley A1/A10/A3 and A4 classes, with regard to the re-numbering sequences of the rest of the LNER locomotive stock there are many sources explaining the logic behind and the anomalies occurring in them.